'Modern Family' Kid -- I Moonlight in Movies for $3k per Week

Ariel Winter -- the girl who plays brainy Alex Dunphy on "Modern Family" -- is a tiny money-making MACHINE ... 'cause besides her steady sitcom paycheck, she made an extra $3k per week to star in a TV movie last year ... and she's only 14-YEARS-OLD!!!!

TMZ has obtained legal docs which show Ariel took a gig back in 2011 appearing in the Nickelodeon TV movie, "Fred 2: Night of the Living Fred" ... during her downtime from "Modern Fam."

According to the docs, Winter raked in $2,808 per week to play the role of "Talia" ... the new girl in school. It's unclear how many weeks she spent on the project.

Meanwhile, Ariel is reportedly making between $10k and $15k per episode on "Modern Family" ... and is expected to renegotiate for A LOT MORE.

And if that wasn't enough ... in 2011 -- when she was only 13 -- she also did some voice work on a couple of Disney cartoons AND a "Final Fantasy" video game.